Water-jet pumps are for example used extensively in laboratories, for example for drying of substances or for evaporating of solutions at a low temperature, through which a thermal load on the substances to be treated is avoided. Water-jet pumps have the advantage that they are built very simply, practically have no wear and are practically insensitive against corrosion, if they are, as common, manufactured of glass or of plastic. A moderate vacuum can be produced without difficulties with water-jet pumps in the range of between 100 mm. mercury column and 15 mm. mercury column, which is sufficient for many laboratory purposes.
Disadvantageous, in comparison with mechanical pumps, for example rotary slide valve pumps, is the relatively high operating cost, since the water usage is considerable; a usual consumption value is for example 0.8 m.sup.3 /h, so that during a ten-hour operation 8 m.sup.3 water are used. Aside from cost, such a high water consumption is also undesirable in view of the environmental resources. To avoid the water consumption, one has already used the above-mentioned method, according to which the water is recirculated in a cycle. For this, an electrically driven recirculating pump is used to place water, which runs off from the water-jet pump, again under pressure and feed it to the operating water connection of the water-jet pump.
The pressure reachable depends strongly on the temperature of the operating water, since the pressure can never be less than the steam pressure of the operating water, which increases with an increase in the temperature of the operating water. The operating water heats up relatively quickly in a recirculating system, so that the suction capacity of the water-jet pump is less than with a constant supply of fresh water. If one still wants to achieve lower temperatures, then the operating water must be cooled with special cooling devices. In this case, a complex apparatus with many electrical connections is the result.
The purpose of the invention is to provide a method of the above-mentioned type such that electrical installations are not needed and still a cooling of the operating water is achieved. Through a further development of the invention an advantageous apparatus for carrying out the method is also to be created.